There is known an OCB-type liquid crystal display having OCB-mode liquid crystal as a liquid crystal display that presents both fast response and high viewing angle.
FIG. 18(a) shows a general structure of the OCB-type liquid crystal display. An OCB-type liquid crystal display 1 comprises an array substrate 2, an opposing substrate 3 opposite to the array substrate 2, and OCB-mode liquid crystal (hereinafter simply referred to as “liquid crystal”) interposed between the array substrate 2 and the opposing substrate 3.
FIG. 19 shows a general cross-sectional structure of the OCB-type liquid crystal display. As shown in FIG. 19, a retardation film 91d and a polarizer 92d are disposed in this order under the array substrate 2. Pixel electrodes 23 and an alignment layer 6d for aligning the liquid crystal in predetermined direction, which are mentioned later, are disposed on the array substrate 2. In the same manner, a retardation film 91u and a polarizer 92u are disposed on the opposing substrate 3. A counter electrode 31 and an alignment layer 6u for aligning the liquid crystal 5 in predetermined direction, which are mentioned later, are disposed under the opposing substrate 3. A polarization axis of the polarizer 92d and a polarization axis of the polarizer 92u are orthogonal to each other. In FIG. 18, for easier understanding, the alignment layers 6, the retardation films 91, and the polarizers 92 are omitted.
As shown in FIG. 18(a), the array substrate 2 has a transparent array substrate body 20, a plurality of source lines 21 provided on the array substrate body 20 and in parallel with longitudinal direction and a plurality of gate lines 22 orthogonal to these source lines 21. There are provided a plurality of transparent pixel electrodes 23 on the array substrate body 20 such that the pixel electrodes 23 are each surrounded by adjacent two sources lines 21 and adjacent two gate liens 22. On the other hand, the opposing substrate 3 has a transparent opposing substrate body 30 and a transparent counter electrode 31 provided over substantially the entire surface of the opposing substrate body 30. In FIG. 18(a), a color filter 32 is interposed between the transparent opposing substrate body 30 and the counter electrode 31, but the color filter 32 may be provided on the side of the array substrate 2.
FIG. 18(b) is an enlarged view of the pixel electrode 23. Each pixel electrode 23 is provided with a switching device 4 comprised of a thin film transistor generally expressed as “TFT”. To be more specific, the switching device 4 comprised of the thin film transistor has a gate electrode 41 connected to the gate line 22, source electrode 42 connected to the source line 21, and a drain electrode 43 connected to the pixel electrode 23. The source electrode 42 and the drain electrode 43 are connected by means of a semiconductor thin film which is not shown. The gate electrode 41 overlaps with the semiconductor thin film with a gate insulating layer (not shown) interposed between them and a drive voltage applied to the gate electrode 41 causes the source electrode 42 and the drain electrode 43 to be switched on through the semiconductor thin film.
An operation of the switching device 4 comprised of the thin film transistor will be described in conjunction with image display. In a normal state, −10V voltages are being applied to the gate lines 22 and the gate electrodes 41 connected to the same. In this state, the switching devices 4 are in OFF state. Subsequently, +10V drive voltage is applied to a gate line 22A in first stage shown in FIG. 18(a), to cause the respective switching devices 4A in first stage to be turned “ON”. This allows the source electrodes 42 and the drain electrodes 43 to be electrically connected. As soon as the switching devices 4 are turned ON all at once, voltages corresponding to an image to be displayed are applied to the respective source lines 21. The voltages applied to the source line 21 are applied to the respective pixel electrodes 23 through the source electrodes 42 and the drain electrodes 43. This generates potential difference between the respective pixel electrodes 23a in first stage and the counter electrode 31.
Subsequently, −10V voltage is applied to the gate line 22A in first stage again to cause the switching devices 4A in first stage to be turned OFF. Simultaneously, +10V voltage is applied to a gate line 22B in second stage to cause switching devices 4B in second stage to be turned ON all at once. In the aforementioned manner, as soon as the switching devices 4B are turned ON, voltages corresponding to an image to be displayed are applied to the respective source lines 21. This generates potential difference between the respective pixel electrodes 23B in second stage and the counter electrode 31.
This operation is repeated for gate lines 22C . . . in third and the following stages, thereby generating potential difference corresponding to an image to be displayed between the respective pixel electrodes 23 and the counter electrode 31. This potential difference causes the liquid crystal 5 to be modulated according to the image to be displayed. Here, a general image display method in the OCB-type liquid crystal display will be described. A lower surface or a side surface of the OCB-type liquid crystal display is irradiated with light from a backlight that is not shown. As shown in FIG. 20, in this light, only light having a polarization plane identical to a polarization axis 921d of the polarizer 92d, passes through the polarizer 92d. Then, this light (polarized light) passes through the retardation film 91d so as to be given retardation (approximately −35 nm) of the retardation film 91d. 
The light, which has passed through the retardation film 91d, is transmitted through the liquid crystal 5 with retardation regulated according to the image to be displayed. This further gives the light retardation. Then, this light reaches the retardation film 91u. The retardation film 91u has the retardation (approximately −35 nm in the above example) equal to that of the retardation film 91d, and still further gives the light retardation.
Here, “retardation” will be explained in detail. As shown in FIG. 20, a polarized light L composed of sine wave, which has passed though the polarizer 92d having a predetermined polarization axis 921d, is considered to be decomposed into two orthogonal sine wave components Lx, Ly. As shown in FIG. 20(a), when longitudinal axis LQLS of the liquid crystal 5 (liquid crystal molecule 51 to be precise) is parallel to axis y along which the polarized light L travels, distance D1 (indicated by bold line in FIG. 20) which the component Lx travels through the inside of the liquid crystal 5 is equal to distance D2 (indicated by bold line in FIG. 20) which Ly travels through the inside of the liquid crystal 5. Since the component Lx and the component Ly exit from the liquid crystal 5 simultaneously, no retardation is generated.
On the other hand, when the longitudinal axis LQLS of the liquid crystal (liquid crystal molecule to be precise) 5 is vertical to the axis y along which the polarized light L travels (in FIG. 20(b), the component Lx is parallel to the longitudinal axis LQLS), the distance D1 which the component Lx travels through the inside of the liquid crystal 5 is longer than the distance D2 which the component Ly travels through the inside of the liquid crystal 5. Therefore, the component Lx exit from the liquid crystal 5 later than the component Ly. Therefore, the sine wave component Lx is behind(on the left side in FIG. 12(b)) the sine wave component Ly. This difference is “retardation”.
As mentioned previously, the liquid crystal has given retardation as the result of modulation according to the image to be displayed. By way of example, in white display, the liquid crystal 5 has retardation of 345 nm, while in black display, the liquid crystal 5 has retardation of 70 nm.
In case of black display, retardation given by the retardation films 91 and the liquid crystal 5 is equal to 0 (=−35 +70 −35), i.e., no retardation is generated. On the other hand, in case of white display, the retardation given by the retardation films 91 and the liquid crystal 5 is 245 nm (=−35 +345 −35) in the above example. Since the polarization axis of the polarizer 92d and the polarization axis of the polarizer 92u are orthogonal to each other, the light which has passed through the retardation film 91u and reached the polarizer 92u, cannot pass through the retardation film 92u because of absence of the retardation, thereby resulting in “black display”. To be a greater detail, as shown in FIG. 20(a), when the retardation between the sine wave components Lx, Ly is 0, polarization plane of the polarized light L resulting from composition of the sine wave components Lx, Ly is parallel to the polarization axis 921d of the polarizer 92d and orthogonal to the polarization axis 921u of the polarizer 92u. Therefore, this light cannot pass through the polarizer 92u, thereby resulting in “black display”.
On the other hand, in case of white display, retardation given by the retardation films 91 and the liquid crystal 5 is 245 nm (=−35 +345 −35) in the above example. When the retardation between the sine wave components Lx, Ly is 245 nm, the polarization plane of the polarized light L resulting from composition of the sine wave components Lx, Ly is parallel to the polarization axis 921u of the polarized light 92u. Therefore; this light passes through the polarizer 92u, thereby resulting in “white display”. While the retardation is set to 245 nm in the above description, the retardation necessary for white display is suitably selected by those skilled in the art.
FIG. 21 is a graph showing luminance—voltage characteristic of the general OCB-mode liquid crystal 5. Under the voltage being increased, the retardation which the liquid crystal 5 gives to polarized light reduces and luminance reduces, which finally results in “black display”. Under the voltage being reduced, the retardation which the liquid crystal gives to polarized light increases and luminance increases, which finally results in “white display”. In this manner, luminance corresponding to an image to be displayed is regulated for each pixel electrode 23.
The light with regulated luminance, is finally transmitted through the color filter 32. As shown in FIG. 22, in a general liquid crystal display, each of laterally arranged color filters of three colors of red(R), green(G), and blue(B) overlaps with one pixel electrode 23 with a correspondence, thereby forming one pixel. One dot is composed of three pixels of three primary colors, i.e., a pixel corresponding to the red color filter, a pixel corresponding to the green color filter, and a pixel corresponding to the blue color filter. Dots composed of the pixels of RGB are provided in a predetermined number, and in front and back direction and right and left direction. For example, when a liquid crystal display has 768 dots in front and back direction and 1076 dots in right and left direction, it has 1076×768×3 (about 250 million) pixel electrodes 23. As a matter of course, in some liquid crystal displays, one dot is composed of three pixels of RGB arranged in longitudinal direction.
As shown in FIG. 23(a), in a non-display state, the OCB-mode liquid crystal 5 has a splay alignment state. The splay alignment state is not suitable for image display. So, before the image is displayed, it is necessary to “initialize” the OCB-mode liquid crystal 5 by application of high voltage to the liquid crystal 5 through the pixel electrodes 23 provided on the array substrate 2 and the counter electrode 31 provided on the opposing substrate 3. This initialization allows the OCB-mode liquid crystal 5 to transition to a bend alignment state as shown in FIG. 23(b). Then, in this bend alignment state, the potential difference generated between the respective pixel electrodes 23 and the counter electrode 31 generates retardation, thereby displaying an image.
One method of initialization is described in Publication of Unexamined Patent Application No. Hei. 10-206822. As shown in FIG. 24, this Publication discloses that voltage Vs of the respective pixel electrodes 23 is held constant, while varying voltage Vcom of the counter electrode 31 like rectangular pulse wave, thereby performing initialization.
In the initialization described in the above-identified Publication, the potential difference by the rectangular pulse wave is generated over the entire liquid crystal 5 disposed between the array substrate 2 and the opposing substrate 3. This is because the voltage Vs of the respective pixel electrodes 23 is constant and the counter electrode 31 is provided over nearly the entire surface of the opposing substrate body. So, in accordance with the initialization disclosed in this Publication, as shown in FIG. 25, the potential difference is generated only in thickness direction of the liquid crystal 5. The potential difference is not generated in right and left direction and front and back direction of the liquid crystal 5. That is, this Publication does not disclose the potential difference in right and left direction and in front and back direction in the initialization of the OCB-type liquid crystal display. The same is the case with PCT/WO00/14597 Publication and Publication of Unexamined Patent Application No. 2001-83552.